Mantle Research Papers - Academia.edu (original) (raw)

by Starostenko, V.I., Janik T., Yegorova T., Farfuliak L., Czuba W., Sroda P., Thybo H., Artemieva I.M., and 11 authors more

Dike-like bodies of garnet (Py 77.04–84.66) hornblendites were recorded, for the first time, in the structurally lowest parts of the gneiss of Meatiq Core Complex in the Central Eastern Desert of Egypt. They are composed mainly of... more

Dike-like bodies of garnet (Py 77.04–84.66) hornblendites were recorded, for the first time, in the structurally lowest parts of the gneiss of Meatiq Core Complex in the Central Eastern Desert of Egypt. They are composed mainly of hornblende (pargasite to pargasitic hornblende), [depleted in LREE with flat HREE segments (normalized to CI-chondrite), and enriched in LILE (normalized to N-MORB)] and crystallized from sub-arc tholeiitic melt that derived from wet mantle wedge (615–600 Ma ago), at P = 23–27 kbar and T = 510–600 °C. Diapirism of hot mantle due to decompression caused partial dehydration melting (P = $12 kbar and T = 800–>1100 °C) of hornblende into pyrope-almandine garnet and diopside (Wo 49– 51 En 32–34 Fs 16–17.5). The clockwise P-T path continued by isothermal decomposition (10–11 kbar) of garnet into hercynite-rich spinel (XFe 2+ = 0.46–0.64) and quartz. By continuous cooling and probably by introduction of the Abu Ziran granite-related solutions, the hornblendites followed a retrograde greenschist facies hydration path, including transformation of diopside to tremolite (T = 280–420 °C and pressure > 5 kb) and garnet to chlorite (T = 190–345 °C). In addition, metasomatic minerals mainly of feld-spars and titanite, and minor ilmenite, rutile, topaz and calcite formed, as well as composition of the original amphibole changed having increases of FeO, TiO 2 and K 2 O and decreases of Al 2 O 3 , MgO and Na 2 O. Deep source of the Meatiq hornblendites implies for a probable local crustal thickening of Meatiq Core Complex by crustal shortening, while their exhumation was most probably accompanied the NW-SE extension and thinning of the previously thickened crust that occurred during oblique island arc convergence with the closure of the Mozambique ocean and the collision of East and West Gondwanaland.

Northwestern Iberia preserves one of the most complete Paleozoic sequences that document the origin and development of a passive margin along the southern (Gondwanan) flank of the Rheic Ocean. In addition to a well preserved sedimentary... more

Northwestern Iberia preserves one of the most complete Paleozoic sequences that document the origin and development of a passive margin along the southern (Gondwanan) flank of the Rheic Ocean. In addition to a well preserved sedimentary record, there is widespread Ordovician volcanic activity that can be used to probe the nature of the lower crust and mantle lithosphere that sourced the volcanic rocks during the Rheic ocean opening. The Ordovician rift-related volcanic sequences provide first-order constraints on the early evolution of the Rheic Ocean. In addition to published and new lithogeochemical data, we provide Sm/Nd isotopic data which together constrain the mantle or crustal source and allow an assessment on the role of the basement in Rheic Ocean magmatism. The data imply that the mafic rocks are derived from a variety of sources, including juvenile mantle that was contaminated by subduction coeval with Early Ordovician magmatism, suggesting the importance of arc activity in northwest Iberia during the opening of the Rheic Ocean. Other basalts were derived from a subcontinental lithospheric mantle that was enriched at about 1.0 Ga. Basalts derived from a mantle enriched at ca. 1.0 Ga occur along other parts of the Gondwanan margin (Avalonia, Oaxaquia) and so the Iberian basalts may be a local representation of a regionally significant enriched mantle. The Sm–Nd isotopic characteristics permit a genetic connection between this mantle source and the basement rocks recently identified in northwest Iberia. Felsic magmas are predominantly intra-crustal magmas derived from melting a Mesoproterozoic crust, lending support to other lines of data that the Gondwanan margin of northwest Iberia was predominantly underlain by a South American (Rio Negro) source.

Exsolution lamellae in pyroxene and garnet porphyroblasts in pyroxenite xenoliths from the Mir, Udachnaya, and Obnazhennaya kimberlites (Siberian Craton) reveal a diverse suite of exsolved phases, including oxides (spinels, ilmenite,... more

Exsolution lamellae in pyroxene and garnet porphyroblasts in pyroxenite xenoliths from the Mir, Udachnaya, and Obnazhennaya kimberlites (Siberian Craton) reveal a diverse suite of exsolved phases, including oxides (spinels, ilmenite, rutile, and chromite), pyroxene, and garnet. Textural characteristics suggest that exsolved phases progressively increased in volumetric proportions, and in some cases, the bulk xenoliths transformed from a lithology dominated by coarse grains (i.e. > 2 cm; megacrystalline) to a significantly finer-grained texture (i.e. < 1 cm).These exsolved lamellae are the result of a complex and protracted sub solidus history following magmatic crystallization. Equilibrium pressure–temperature estimates place these xenoliths at low-to-moderate pressure–temperature conditions (690–910°C and 2.0–4.5 GPa) in the lithospheric mantle at the time of entrainment in the kimberlite. However, reconstructed compositions of initial pyroxene and garnet crystals suggest that this suite of pyroxenites formed at considerably higher temperatures and pressures that, in some instances, may have approached the majorite stability field. Pyroxenites that do not contain primary garnet may have been derived from shallower depths.Progressive exsolution in these pyroxenites is of importance inasmuch as such processes can permit localized changes in rheological properties and may also accommodate strain within portions of lithospheric mantle. Because most xenolith studies focus on peridotites and eclogites, the pyroxenite sample suite studied in this work represents an important contribution towards a greater understanding of the Siberian lithospheric mantle.

The Galápagos Spreading Center (GSC) is marked by systematic changes in axial morphology between the Inca Fracture Zone (FZ) at 85.5°W and the 95.5°W propagator. We analyze these changes using new swath bathymetry and magnetic data... more

The Galápagos Spreading Center (GSC) is marked by systematic changes in axial morphology between the Inca Fracture Zone (FZ) at 85.5°W and the 95.5°W propagator. We analyze these changes using new swath bathymetry and magnetic data acquired aboard the B/O Hespérides during the Galápagos'96 experiment. Within ˜350 km of the Galápagos hotspot the ridge axis is associated with an East Pacific Rise (EPR)-like axial high. At increasing distance from the hotspot the axial high broadens and deepens forming a distinctive transitional axial morphology (TAM). The axis in this transitional region is typically a broad zone (˜20 km wide) consisting of very rough volcanic and fault-generated topography. West of 95°W, this TAM evolves into a 20-40 km wide, 400-1500 m deep axial valley typical of the slow spreading Mid-Atlantic Ridge (MAR). There is not an abrupt change from axial high to rift valley along the GSC, but a distinct TAM occurs over a distance of ˜200-300 km along-axis and is accompanied by a gravity-estimated crustal thickening of >1-2 km. The boundary between an axial high and this TAM is quite abrupt and occurs along a segment that is less than 9 km long. These changes in axial morphology are primarily caused by variations in magma supply along the GSC due to the entrainment and dispersal of plume mantle from the Galápagos hotspot. However, the changes in morphology are not symmetric about the Galápagos FZ at 91°W. The axial high topography extends farther east of the 91°W FZ than to the west, and the rift valley which develops west of 94°W is not found at comparable distances along the GSC east of the hotspot. Axial depth variations are also asymmetric across the 91°W FZ. This asymmetry in both morphology and axial depth variation is attributed to a full spreading rate increase along the GSC from 46 mm/yr at 97°W to 64 mm/yr at 85°W. Off-axis depth changes are symmetric about the 91°W FZ and suggest that 15-40% of on-axis depth variation is dynamically supported.

The article considers textile findings of the 17th-18th centuries obtained in the course of archaeological works in 2014-2015 at the cemetery of the Church of St John the Baptist in the cloister of Novodevichy Convent. The items were in a... more

The article considers textile findings of the 17th-18th centuries obtained in the course of archaeological works in 2014-2015 at the cemetery of the Church of St John the Baptist in the cloister of Novodevichy Convent. The items were in a highly unsatisfactory condition in terms of preservation. After their studying it was possible to determine their purpose and partially reconstruct the appearance and decoration. The most interesting remains were fragments of shirts, covers and vestment mantle.

A borehole drilled at Fiumicino (Rome) down to only 27 m depth in a zone where no gas emission at the surface was known, caused a gas blowout from a pressurized gas pocket confined beneath a clay cover. Gas slowly diffused from the... more

A borehole drilled at Fiumicino (Rome) down to only 27 m depth in a zone where no gas emission at the surface was known, caused a gas blowout from a pressurized gas pocket confined beneath a clay cover. Gas slowly diffused from the borehole within superficial permeable sand. Seven persons living in three ground floor flats of a near building had

Simple arguments show that ascending thermal plumes will entrain their surroundings as the result of coupling between conduction of heat and laminar stirring driven by the plume motion. In the initial stages of ascent of a plume fed by a... more

Simple arguments show that ascending thermal plumes will entrain their surroundings as the result of coupling between conduction of heat and laminar stirring driven by the plume motion. In the initial stages of ascent of a plume fed by a continuous buoyancy flux (a starting plume) the plume consists of a large buoyant head followed by a narrow vertical conduit.

Assessment of potential future eruptive behaviour of volcanoes relies strongly on detailed knowledge of their activity in the past, such as eruption frequency, magnitude and repose time. The eruption history of three partly subglacial... more

Assessment of potential future eruptive behaviour of volcanoes relies strongly on detailed knowledge of their activity in the past, such as eruption frequency, magnitude and repose time. The eruption history of three partly subglacial volcanic systems, Grímsvötn, Bárdarbunga and Kverkfjöll, was studied by analysing tephra from soil profiles around the Vatnajökull ice-cap, which extend back to ~7.6 ka. Well known regional Holocene marker tephra (e.g. H3, H4, H5) were utilized to correlate profiles. Stratigraphic positions and geochemical compositions were used for fine-scale correlation of basaltic tephra. Around Vatnajökull ice-cap 345 tephra layers were identified, of which 70% originated from Grímsvötn, Bárdarbunga or Kverkfjöll. The eruption frequency of each volcanic system was estimated; Grímsvötn has been the most active with an average of ~7 eruptions/100 years (range 4–14) during prehistoric time (before ~870 AD); Bárdarbunga has been the second most active with ~5 eruptions/100 years (range 1–8); and Kverkfjöll has remained essentially calm with 0–3 eruptions/100 years but showing periodic activity with repose times of >1000 years. All three volcanic systems experienced lulls in activity from 5 ka to 2 ka, referred to as the “Mid-Holocene low”. This reduced eruption frequency appears to have resulted from a decrease in magma generation and delivery from the mantle plume rather than from changes in ice-load/glacier thickness. In prehistoric time, there was a time lag of 1000–3000 years between a peak of activity at volcanoes directly above the mantle plume versus at volcanoes located in the non-rifting part of the Eastern Volcanic Zone, closer to the periphery of the island. This time-space relationship suggests that a significant future increase in volcanism can be expected there, following increased levels of volcanism above the plume.